High speed band type computer impact printers utilize as many as 150 pushrods each adapted to actuate an impact hammer. These pushrods operate at up to 50 impacts per second and at operating temperatures up to 190.degree. F. The type of pushrods currently in use is shown in FIG. 1 and designated "Prior Art". It comprises an elongated pushrod wire 1 having a tip member 2 at each end thereof, the tip members usually being of identical configuration. Each pushrod tip comprises a body 3 having a shallow cup-shaped recess 4 formed in its outer face, the body terminating at its opposite face in a cylindrical shank 5 having a blind bore 6 extending inwardly from the distal end of the shank, the bore terminating short of the cup-shaped recess 4. Recess 4 is filled with an elastomeric material which forms a relatively shallow impact button 7. The impact buttons have heretofore been formed by placing a drop of a liquid elastomer in the recess 4 and permitting it to cure in situ, the elastomer being admixed with a curing agent. The contour of the exposed portion of the impact button is determined by the surface tension of the elastomer, such contour being of essentially semi-hemispherical shape, as indicated at 8. The tip members 2 are fixed to the wire 1 by inserting the opposite ends of the wire into the bores 6 of the tip members and swaging the shanks 5 to the wire, the ends of the wires being first roughened by means of a knurling tool. The current practice is to swage the opposite sides of the shank 5 so as to compress the shank so that it has a somewhat eliptical shaped cross-section.
While pushrods of the types described have been successfully used, they are subject to failure and are expensive to manufacture due to their configuration and the manner in which the impact buttons are formed. Failures occur due to the fact that the tip members lose their impact buttons which are secured to the tip members solely by the adhesive bond between the elastomeric material and the shallow recess in the tip member. It also may be noted that the adhesion of the elastomeric material to the tip members requires elaborate surface preparation of the tip members prior to application of the elastomer. Since the shape of the external surface of the impact buttons is primarily determined by the free surface tension of the elastomeric material from which they are formed, the tip profile is essentially semi-hemispherical and the initial impact area of the buttons is quite small and immediate movement and compliance of the elastomer is necessary during initial use, resulting in some degree of permanent set of the elastomer. During the initial burn-in period of the printers it becomes necessary to compensate for this set in the elastomer resulting from applied impact forces. Due to the configuration of the prior art impact buttons, the impact force deformation of the elastomer is inherently directed radially outwardly which, coupled with the relatively shallow configuration of the recesses, results in high unit impact stresses within the impact buttons and they are more readily susceptible to permanent deformation under load.
Another problem encountered with presently known pushrods is their tendency to fail by fracture and breakage of the pushrod wires, generally in the areas immediately adjacent the cylindrical shanks 5. Breakage of the pushrod wires is believed to be due in part to the configuration of the impact buttons and the manner in which they transmit the impact forces to the wire rods. Wire breakage also may be caused by the manner in which the shanks of the tip members are swaged to the wire, the customary procedure being to employ an opposing pair of staking members which form the opposite sides of the shanks. Staking in this manner has a tendency to cause directional deflection of the tip members in use and loss of concentricity with respect to the pushrod wires.
The present invention overcomes the inherent deficiencies in presently used pushrods by providing a pushrod construction in which the configuration of the impact buttons and the manner in which they are secured to the pushrod tips effectively prevents the loss of the impact buttons as well as undesirable deformation of the buttons and the resultant severity of the impact forces transmitted to the pushrod wires.